[SI-LIST] Re: 60 MHz sinusoidal clock Decoupling
- From: "Steve Rogers" <SRogers@xxxxxxxxxxxxx>
- To: <cchalmers@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 13 Apr 2004 17:00:32 +0100
First in general terms..........the decoupling network you need on any =
circuit really depends on
(a) how much rubbish thats on the supply you can allow into the circuit.
(b) How much of the rubbish from the ciruit (60 MHz plus harmonics in =
this case)you can allow back into the supply.
With many circuits and systems you can get away with quite poor =
decoupling such as the old "One 100nF cap per chip and a larger cap for =
bulk decouple". I would not recommend this approach under any =
circumstances. Its better to think about the potential frequencies you =
will have and how much of these you can stand in each part of the =
system. You then sort out the decoupling to achieve your limits.=20
You dont say what the 60 MHz sine wave clock will be used for. If its in =
a radio application you will really need to make sure that its supply is =
clean or any rubbish on the supply will modulate the clock and end up on =
your output.=20
Looking at things the other way, if your clock leaks onto the power =
supply lines and these lines have some length to them or have nice loops =
then you have some nice radiated emissions to sort out.
If you can get away with it a series resistor is always nice to give you =
R-C as opposed to just R plus some undefined trace inductance. If your =
supply currents are too large for any significant R then you can go to a =
ferrite bead or lossy inductor.
Finally, some nasty things can happen with poor decoupling such as =
instability (spurious oscillations from your 60 MHz oscillator at some =
frequency you didnt want).=20
Now, more specifically to your question........... You are right that =
the 100nF cap being self resonant at 12 Mhz is not great. Its going to =
be looking like an inductor between the supply and ground. This is not =
ideal. You will often see designs with a single decoupling cap selected =
by its SRF such that it is near to the operating frequency. In this way =
you really screw the power supply down to ground at this frequency. =
However, in practice you should really be looking at a couple of =
capacitors, one for the op frequency of 60 MHz and then a larger value =
to cover the lower frequency stuff that could get onto your supply. You =
can then do some modelling or better still just prototype it and make =
some measurements.
=20
=20
Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer
Micromill Electronics Limited
Leydene House
Waterberry Drive=20
Waterlooville Hampshire
PO7 7XX
Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673
Registered No. 1456922 (England). =20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ
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-----Original Message-----
From: Chris Chalmers [mailto:cchalmers@xxxxxxxxxxx]
Sent: Tuesday, April 13, 2004 3:51 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 60 MHz sinusoidal clock Decoupling
SI,
This is theoretical. Is there any point in
putting standard 0603 0.1uF capacitors (SRF ~ 12MHz) on to
decouple a clock oscillator that is producing a pure
60MHz Sine wave. To my mind there would be no point
because 0.1uF caps won't have the low impedance we
want at that frequency. Is this correct?
Thanks in advance
Chris
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